I'm not sure why YouTube embeds wouldn't display, unless there was some issue fetching the thumbnail at the time of upload.
If the images are especially large, then they may be tripping the maximum image resolution limit. Images which are over 20 megapixel, by default, we don't attempt to resize them due to the potential to hit various memory limits.
If memory isn't an issue, you can raise this limit in config.php:
$config['maxImageResizePixelCount'] = 30000000;
You can rebuild all media thumbnails in Admin CP > Applications > Gallery Rebuilds